SYSTEMS FOR AIR FLOW MANAGEMENT IN INKJET PRINTERS

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Spanish Patent Application No. P202430735 filed Sep. 18, 2024, the content of which is herein incorporated in its entirety.

TECHNICAL FIELD

Various of the disclosed embodiments concern systems for air flow management in inkjet printers.

BACKGROUND

Inkjet printing is a non-contact process based on ejecting inkjet droplets from a printhead. These droplets travel through a small distance, the printhead-substrate gap, and are deposited on a receiving substrate to form the desired image. Different methods for generating the drop ejection can be found. For example, piezoelectric printheads induce the drop ejection by generating pressure waves through the deformation of a piezoelectric crystal, thermal printheads induce the drop ejection by locally vaporizing a small quantity of ink that upon expansion release liquid ink, etc.

During the drop ejection process, small secondary droplets are sometimes generated. These are typically called satellite droplets. The formation of these droplets is linked to the complex interplay between inertial and surface tension forces just after drop ejection. These droplets are very small and are ejected at low velocity, resulting in a very small probability of them falling on the desired location. As a result, wandering droplets can fall on undesired locations of the substrate or on the surface of the printhead nozzle plate. This results in a degradation of printing quality and printer robustness because additional cleaning procedures are required.

One common solution to this problem, introduced in industrial printers, involves the integration of a vacuum system in the vicinity of the printheads. This vacuum system generates a mild air flow in the gap between the printhead and the substrates that removes small satellite droplets present in this region and prevents these droplets from falling on undesired locations. Such vacuum generation is typically accomplished by using a blower or a compressed air-based vacuum system. Additionally, liquid removal elements may be present to prevent the removed fluid from getting into the vacuum generation system.

SUMMARY

In embodiments of the invention a single vacuum source is used for multiple high flow rate subsystems in an inkjet printer to control the dispersal of satellite droplets during ink ejection while printing. This reduces cost and layout requirements, provides a simplified control structure, and integrates the vacuum source into a centralized liquid removal system. Accordingly the printing quality, robustness, and versatility of inkjet printers is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a single-pass printing machine, showing a schematic view of the vacuum table and mist removal elements according to embodiments of the invention;

FIG. 2 is a side view of a single-pass printing machine, showing a schematic view of the vacuum table and mist removal elements according to embodiments of the invention;

FIG. 3 is a block diagram describing the air flow path in a combined vacuum system according to embodiments of the invention;

FIG. 4 shows a mist removal system suction element with control valve according to embodiments of the invention;

FIG. 5 shows a vacuum control system for keeping flow rate constant in a mist removal system under a variable downstream vacuum level according to embodiments of the invention; and

FIG. 6 is a block diagram of a computer system as may be used to implement certain features of some of the embodiments.

DETAILED DESCRIPTION

In state of the art industrial printers, multiple air flow sources for different requirements may be necessary. These include air flow sources for vacuum table conveyor systems, mist removal systems, printhead cleaning systems, substrate cleaning systems, substrate cooling systems, etc. Typically, a separate air flow source is used for each of these subsystems resulting in higher energy consumption, maintenance requirements, layout/packaging requirements and initial and operation costs.

Embodiments of the invention are applicable to digital inkjet printing machines that include a vacuum conveyor system for transporting the substrates and a vacuum system for removing satellite drops induced by the printing process. These two subsystems have certain aspects in common:

• • They are relatively high conductance, which means that they can produce relatively high flow with low vacuum depression levels;
  • They share a similar region in the printer because the mist removal system is placed just above the substrate while the vacuum table system is placed just below the substrate; and
  • Both induce a multiphase flow of dispersed ink droplets in a main air stream.

FIG. 1 is a front view of a single-pass printing machine, showing a schematic view of the vacuum table and mist removal elements according to according to embodiments of the invention. In FIG. 1, a series of print bars 10 have corresponding mist removal system suction elements 12 positioned proximate thereto. A substrate 14 is conveyed past the print bar and mist removal system suction elements by a vacuum table conveyor system 16. Both the suction elements and the vacuum table conveyor system may use a vacuum source during operation. The mist removal system suction elements draw satellite droplets away from the substrate when ink is ejected from the print bars and thus prevents these droplets from falling on undesired locations.

The mist removal system may alternatively use a separate blower or other such vacuum generation mechanism for preventing the satellite droplets from falling onto the surface of the substrate.

FIG. 2 is a side view of a single-pass printing machine, showing a schematic view of the vacuum table and mist removal elements according to embodiments of the invention. In FIG. 2, a print bar 10 is shown relative to the mist removal system suction elements 12. A connector tube 20 provides a vacuum source to the mist removal system suction elements.

As shown in FIG. 2, the substrate 14 is conveyed past the print bar and mist removal system suction elements by a vacuum table conveyor system 16. In embodiments of the invention, the vacuum source 24 to which the connector tube is connected is shared with the vacuum source for the vacuum table conveyor system.

Embodiments of the invention involve the use of a single vacuum table source for both the mist removal and vacuum table conveyor systems. This leads to a reduction in cost and layout requirements, a simplified control structure, and the capacity to integrate a single centralized liquid removal system 22. In embodiments of the invention the centralized liquid removal system is composed of a combination of coalescing, separating, e.g. a cyclone, and filtrating elements to remove ink from the air stream. These are the elements that compose the liquid removal system that induces the separation of the liquid from the main air stream. The system prevents damage of the vacuum source by preventing ink from getting to it.

FIG. 3 is a block diagram describing the air flow path in a combined vacuum system according to embodiments of the invention. In FIG. 3, the air flow comes from the printer surrounding atmosphere 30. The vacuum table system 31 and the mist removal system 32 both receive a vacuum for operation from a common vacuum source 35. Examples of vacuum sources include a centrifugal fan, an axial fan, a Venturi-based vacuum ejector, etc. In embodiments of the invention the mist removal system includes a constant vacuum actuator 33 to ensure that its vacuum level is constant irrespective of the vacuum level of the vacuum source. A liquid removal system 34 located before the vacuum source draws such liquid from the system as is captured by the mist removal system, as well as any moisture that may fall on the vacuum table conveyor system as the substrate is moved across its surface. The system is open to the atmosphere 36. The air flow is typically exhausted back to the surrounding atmosphere once it passes through all the elements of the pneumatic system after the vacuum source.

One important aspect for making this combined system operate properly is ensuring that the desired vacuum levels are maintained in each of the subsystems. Typically the desired vacuum level of the vacuum table conveyor system is modified according to the requirements of the substrate, with more warped or stiffer substrates requiring higher vacuum levels while the mist removal system requires stable vacuum levels to ensure that only the smallest satellite drops are removed while the main drops are not disturbed. The desired vacuum level in each of the subsystems is highly dependent on parameters such as the stiffness and warp level of the substrates, the geometry of the vacuum table conveyor and the mist removal system, the size of the drops, etc. but generally ranges between 0.5 kPa and 8 kPa.

In a state of the art system having a single vacuum source and no additional actuators, it is not possible to maintain stable control of the vacuum level in the mist removal system because the increase in the vacuum level of the vacuum source required to ensure appropriate flatness of the substrate through the vacuum conveyor system also leads to an increase in the vacuum level in the mist removal system. In embodiments of the invention different methods can be integrated for this purpose.

A first method involves having a high enough conductance in the mist removal system to achieve the desired flow at the minimum vacuum power and then having a vacuum relief valve or vacuum regulator at the exit of the mist removal system to admit atmospheric air when high vacuum table vacuum levels are required.

A second method involves having a closed-loop control of the vacuum at the exit of the mist removal system by modifying the conductance of a valve. In high vacuum power conditions the valve is partially closed to prevent excessive flow rate and in low vacuum power conditions it is opened to ensure that the desired flow rate is achieved.

FIG. 4 shows a mist removal system suction element with control valve according to embodiments of the invention. In FIG. 4, a suction element 40 receives a vacuum which is used to create an upward air flow 42 that draws satellite droplets ejected from the print bars away from the substrate. The vacuum is controlled by a control valve 44, in this specific case a butterfly valve that is operated to maintain the vacuum pressure at a desired level based on vacuum pressure at the suction element as sensed by a pressure sensor at a pressure monitoring port 46.

FIG. 5 shows a vacuum control system for keeping flow rate constant in a mist removal system under a variable downstream vacuum level according to embodiments of the invention. In FIG. 5, the system is provided with a vacuum setpoint 50, typically in the range of 0.5 kPa to 8 kPa. The vacuum setpoint is the desired vacuum level. The vacuum setpoint is adjusted to maintain an appropriate vacuum to operate the mist removal system by finding the optimum balance between removal of the satellite and a vacuum error 52 is determined. The vacuum error is provided as an input to a controller 53 to set a desired value angle position 54 for the control valve 44 (see FIG. 4) as effected by a valve position driver 55 to bring the pressure reading to the vacuum level setpoint. The valve position driver 55 is a generic actuator. e.g. it can be electric, pneumatic, hydraulic, etc., that modifies the position of the valve according to what is prescribed by the controller. The valve angle can be measured using different instruments, e.g. an encoder, resolver, a distance sensor, Hall sensor, etc.

Computer Implementation

FIG. 6 is a block diagram of a computer system as may be used to implement certain features of some of the embodiments. The computer system may be a server computer, a client computer, a personal computer (PC), a user device, a tablet PC, a laptop computer, a personal digital assistant (PDA), a cellular telephone, an iPhone, an iPad, a Blackberry, a processor, a telephone, a web appliance, a network router, switch or bridge, a console, a hand-held console, a (hand-held) gaming device, a music player, any portable, mobile, hand-held device, wearable device, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine.

The computing system 300 may include one or more central processing units (“processors”) 305, memory 310, input/output devices 325, e.g. keyboard and pointing devices, touch devices, display devices, storage devices 320, e.g. disk drives, and network adapters 330, e.g. network interfaces, that are connected to an interconnect 315. The interconnect 315 is illustrated as an abstraction that represents any one or more separate physical buses, point to point connections, or both connected by appropriate bridges, adapters, or controllers. The interconnect 315, therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus or PCI-Express bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), IIC (12C) bus, or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus, also called Firewire.

The memory 310 and storage devices 320 arc computer-readable storage media that may store instructions that implement at least portions of the various embodiments. In addition, the data structures and message structures may be stored or transmitted via a data transmission medium, e.g. a signal on a communications link. Various communications links may be used, e.g. the Internet, a local area network, a wide area network, or a point-to-point dial-up connection. Thus, computer readable media can include computer-readable storage media, e.g. non-transitory media, and computer-readable transmission media.

The instructions stored in memory 310 can be implemented as software and/or firmware to program the processor 305 to carry out actions described above. In some embodiments, such software or firmware may be initially provided to the processing system 300 by downloading it from a remote system through the computing system 300, e.g. via network adapter 330.

The various embodiments introduced herein can be implemented by, for example, programmable circuitry, e.g. one or more microprocessors, programmed with software and/or firmware, or entirely in special purpose hardwired (non programmable) circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more ASICs, PLDs, FPGAs, etc.

The various embodiments introduced herein can be implemented by, for example, programmable circuitry, e.g. one or more microprocessors, programmed with software and/or firmware, or entirely in special purpose hardwired (non-programmable) circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more ASICs, PLDs, FPGAs, etc.

The language used in the specification has been principally selected for readability and instructional purposes. It may not have been selected to delineate or circumscribe the subject matter. It is therefore intended that the scope of the technology be limited not by this Detailed Description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of various embodiments is intended to be illustrative, but not limiting, of the scope of the technology as set forth in the following claims.